Colored plastic articles can be prepared by means of incorporating (e.g., by means of compounding) pigments and/or dyes directly into the polymeric materials from which the articles are prepared. Such direct incorporation methods result in the colorant being dispersed substantially throughout the molded article. Direct incorporation methods are not particularly well suited for the preparation of molded articles that are only slightly colored or tinted (e.g., less than opaque), such as in the case of sunshade lenses. It is typically difficult to adequately and sufficiently disperse the small amounts of colorant required to prepare slightly colored or tinted plastic articles, by means of direct incorporation methods.
While master-batches of dye and resin can be used to better control the amount of dye incorporated during compounding and/or extrusion processes, the preparation of the master-batch requires additional steps. In addition, the resin of the master-batch will be exposed to at least two heat cycles (one during the preparation of the master-batch, and another during preparation of the dyed molded article), which can result in a final molded article having degraded physical properties.
The preparation of colored plastic articles by means of applying a dye composition to the surface of the plastic article is generally known. Such dyeing methods are more suited to the preparation of slightly colored or tinted molded articles, as only small and controlled amounts of colorant are incorporated into the surface thereof. The dye compositions may be aqueous or non-aqueous.
In light of environmental concerns related to the use of organic solvents, more recently there has been increased emphasis towards the development of dyeing methods that make use of aqueous dye compositions. Methods of dyeing plastic articles that make use of aqueous dye compositions typically suffer from disadvantages that include, for example, non-uniform and/or inadequate dyeing of the article, and an inconsistent degree of dyeing between different batches of the same plastic articles.
It would be desirable to develop new methods of dyeing plastic articles that make use of aqueous dye compositions, and that result in the formation of uniformly and sufficiently dyed articles. In addition, it would be desirable that such new methods also provide consistent degrees of dyeing over time.